The present invention relates generally to power supplies for use in gas ion laser systems and more particularly to an overall laser system in which a universal laser power controller drives a gas ion laser tube. Specific apparatus and methods are introduced with regard to tube start-up, current sensing for feedback purposes and switching regulation.
A typical gas ion laser system includes a power supply which drives a gas ion laser tube. It should be appreciated that a wide variety of gas ion laser tubes are available depending upon the intended application. Differences between available tubes include cathode voltages ranging from 70 volts DC to 200 volts DC, output power and output wavelength. At the same time, laser tubes having different cathode voltages may require other drive voltages which are identical. For example, most gas ion tubes require a 3.3 volt AC filament voltage.
In the prior art, based on the wide range of available gas ion laser tubes, power supplies have typically been provided in configurations for driving one, specific tube out of the array of available laser tubes using one particular AC input voltage. In view of this approach, manufacturers have been burdened with producing an array of power supplies which is at least as broad as the array of available gas ion laser tubes based solely on tube drive requirements. The number of different power supplies is further proliferated by public utility systems which provide either 110 volts AC or 220 volts AC input. For all of these reasons, distributors and end users of laser systems are likewise burdened by costs associated with the need for a different power supply each time a laser tube with a different drive requirement is used or a different input AC voltage is provided. Thus, as will be described in detail below, an approach is needed for reducing the number of different power supplies required for available laser tubes. Preferably, the approach should also address drive requirements of tubes to be developed in the future.
Other concerns arise with regard to prior art gas ion laser systems. One concern relates to regulation of power output of the laser tube. Regulation is critical not only for controlling the output light level of the laser, but also in regard to maximization of the lifetime of a laser tube. For example, overdriving a tube by only 10% will result in a subsequent shortening of the tube's lifetime by approximately 25%. Prior art regulation arrangements normally utilize a shunt resistor located in the cathode lead of the laser tube. Unfortunately, as will be further described, this arrangement is disadvantageous since filament current provided to the laser tube also circulates in the cathode lead resulting in variations in the voltage sensed by the shunt resistor which variations are not necessarily related to the anode to cathode current passing through the tube. Prior art regulation schemes are also disadvantageous with regard to the typical arrangement of the switching device used in the buck power stage of the power supply. Typically, switching is accomplished using a MOSFET which is located in the "high" side lead (i.e., providing anode current) of the power supply. Unfortunately, producing a precise drive signal for the gate of the MOSFET is quite difficult since the drive signal itself must essentially float on a large DC bias. The DC bias varies from the primary ground to the anode voltage of the laser tube and has a significant impact on MOSFET switching.
Another concern arises with regard to new and rigorous standards (as of the date of this writing) imposed on operation of electrical devices. More specifically, a European standard which is referred to as IEC imposes a restriction on total harmonic distortion of no more than 3% induced into an AC supply line. In this regard, gas ion laser systems are prone to producing such harmonics due to substantial input power requirements. The need for high input power levels is due to the relatively low efficiency of laser systems in converting AC input power to laser light output power. Hence, an alternating current input providing power to the laser system is generally loaded to near its maximum current capabilities. In the instance of such high current loading, prior art laser systems generally introduce significant harmonics which violate the IEC standard. Violation can occur at startup due to inrush currents, and may continue during steady state operation of the laser.
Still another concern relates to line voltage available to power the laser system. In particular, variations in the input voltage provided by a utility may translate into variations in the filament voltage which is problematic if the filament of the laser tube is overdriven since the lifetime of the laser tube is shortened.
The present invention provides a highly advantageous power controller within an overall laser system and an associated method. The power controller is readily configurable in a way which has not been seen heretofore in laser applications for use with different AC input voltages in driving a wide range of gas ion laser tubes having different drive requirements. The overall design features conformance with the aforementioned IEC standard, a highly advantageous startup arrangement and a highly advantageous feedback arrangement, all of which cooperatively ensure consistent startup and stable operation. An auto-ranging AC filament generation arrangement is also introduced.